1. Field of the Invention
This invention relates generally to measuring instruments and more particularly is directed towards a new and improved bore guage head assembly for use with a mechanical or electronic read-out device such as a dial indicator or a digital display coupled to an LVDT, for example.
2. Description of the Prior Art
A bore gauge is an instrument designed to measure inside diameters of a cylindrical bore formed in a variety of different workpieces. By way of example, a machinist drilling or cutting a bore in a workpiece will normally perform frequent precise measurements of the diameter of the bore to ensure that the finished diameter corresponds with specifications. While calipers can be used for this purpose, bore gauges are more convenient and quicker to use. A typical bore gauge used for this purpose is provided with a head having a pair of angularly spaced centralizing contacts, one on either side of a centerline perpendicular to the longitudinal axis of the instrument. At one end of the centerline is an extensible stem and at the opposite end is a sensing contact. The centralizing contacts and the stem serve to position the head so that the centerline is located precisely along the diameter of the bore. In that position the sensing contact, which can move within a limited range of motion, can make a measurement precisely at the diameter of the bore. The sensing contact is operatively connected to a mechanism which drives a dial indicator, LVDT or the like.
Since the sensing contact of a bore gauge typically has relatively small range of motion, the effective measurement range of the instrument can be increased or decreased by providing an extensible stem directly opposite the sensing contact of the gauge head. Such stems heretofore have been in the form of plain screws that are turned in or out as required with particular settings being established by inserting the head of a bore gauge in a reference ring selected to correspond approximately with the diameter of the bore to be measured. Once the bore gauge has been set in the reference ring, the head of the instrument is then inserted in the bore itself and the reading is made, typically on a dial indicator mechanically connected to the measuring head.
While this type of instrument is known in the art as exemplified by the following U.S. Pat. Nos. 3,418,720, 3,442,020, 3,422,540, 4,030,202 and 4,170,831, these instruments have been somewhat difficult to operate and maintain. For example, the initial setting of the extensible stem is quite slow and may require several adjustments before the stem is correctly extended and locked.
In the above identified parent application there are various improvements in bore gauges, these improvements including a stem which is adjusted by means of a rack and pinion drive, floating ball contacts which bear directly against flat faces of the center part of the head and the bore wall during a centralizing manipulation of the instrument and an improved motion between the sensing contact and a transfer rod which drivingly connects with a dial indicator or the like.
It is an object of the present invention to provide further improvements in head assemblies for bore gauges.
A more specific object of this invention is to provide a bore gauge head assembly of simple, rugged construction adapted for hard use without impairing the operation thereof.
Another object of this invention is to provide a head assembly for a bore gauge in which a ball contact serving as the sensing contact may be easily replaced or adjusted. Still another object of this invention is to provide a bore gauge head assembly in which the stem is easily adjusted and provides a direct reading of the stem position.
Still another object of this invention is to provide a bore gauge head assembly which is substantially tamper proof.